Steerable Gas Turbodrill

ABSTRACT

A gas turbodrill with an adjustable bent housing for use in a spur lateral drilling application. The gas turbodrill includes a high-speed gas turbine, a gearbox assembly, a pivoting shaft connection point, a gimbal assembly comprising a hollow ball and socket joint, a bearing assembly and drill bit assembly. The gas turbodrill gimbal assembly enabling a bend through an angle of up to 5 degrees while drilling. Springs and the application of pressure will lock the bend in place once drilling commences to facilitate lateral drilling of the spur.

RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication, Ser. No. 61/643,145 filed on May 4, 2012 all of which isherein incorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to gas turbodrills for downhole drillingoperations.

2. Description of the Related Art

It is generally desirable to operate a drill motor on dry gas forcompletion drilling of water sensitive formations. However, some typesof drill motors are not suitable for this purpose. For example,progressive cavity motors incorporate elastomeric stators that canrapidly degrade when operated on dry gas. Turbodrills are capable ofoperation on dry gas, but these tools stall easily when operated on gas,and the motor speed is generally much too high for effective drilling.Typical turbodrill motors also tend to be very long, which limits thesteer-ability of the drill string. In a paper entitled, “Downholepneumatic turbine motor: testing and simulation results,” SPE DrillingEngineering, September pp 239-246, Lyons et al. describe the developmentand testing of a gas turbine motor for drilling. As described in thispaper, the gas turbine motor included a single stage radial-flow turbineoperating at extremely high rotary speed (i.e., at more than 100,000rpm) and a multi-stage planetary transmission to reduce the speed andincrease torque to the level needed to drive a conventional roller conedrill bit. There are technical challenges that arise when exiting anopen hole during completion drilling, which include:

1. Orientation of the lateral bores in vertical, inclined, or horizontalwells;

2. The kickoff of the lateral;

3. Transport of cuttings away from the drill bit;

4. Hole stability; and

5. Trajectory control.

Coiled tubing drilling (“CTD”) systems capable of sidetracking anddrilling multiple lateral bores are available. These systems have beenused extensively in Alaska to access compartmentalized oil reservoirs.The cost of a CTD bottom hole assembly (“BHA”) includingmeasurement-while-drilling and downhole bit face orientation tools isrelatively high, as is the cost of the surface equipment required tosupport this apparatus. These systems offer full steer-ability andtracking and are capable of drilling at up to 50°/100 ft dogleg severity(“DLS”). [DLS is a normalized estimate of the overall curvature of awell path between two consecutive directional survey stations, accordingto the minimum curvature survey calculation method.] A conventional CTDsystem incorporates a positive displacement motor (“PDM”) designed tooperate on drilling mud. This system develops significant torque andrequires constant trajectory measurement using measurement whiledrilling tools and steering adjustment using a downhole orienter. Thesesteering systems are complex and expensive and greatly increase thelength of the BHA. Wire in coil systems can be required for operation ondry gas since mud pulse telemetry is not feasible when running dry gas.

It would be desirable to develop a steerable gas turbodrill (“SGTD”)that enables high-power, high-rotary speed drilling at a lower torquethan a PDM system and which requires minimal steering, once the SGTD isproperly oriented. This approach would eliminate the need for high-costmeasurement while drilling and the need for bit face orientation systemsin the bottomhole assembly. This tool should be relatively compact andcapable of being readily steered, for example, at least through a 200ft. lateral arc having a constant 120 ft. radius, i.e. a spur lateral.

It would further be desirable to employ a SGTD that uses dry nitrogen,and which includes a gear box, enabling operation at a high rotaryspeed, for efficient power conversion, and but achieving a lowerrotational speed on the output of the gear box, than is possible for agas turbine power section.

SUMMARY OF THE INVENTION

In accordance with the present invention, the problems discussed aboveare solved by a gas turbodrill that includes a drill-bit section, abearing assembly, a gearbox assembly, a gimbal assembly, a high-speedgas turbine power section and a flexible tubing string are fed downholeat the end of a string of pipe for a spur lateral drilling application.

In one embodiment of the invention, the high-speed gas turbine powersection in the upper section of the gas turbodrill rotates a flexibleshaft that extends through a gimbal assembly. The lower section of theturbodrill then contains the gearbox assembly, bearing assembly anddrill-bit section. The gimbal assembly serves as a flex joint for theentire gas turbodrill, which allows the drill to move at an angle awayfrom the central wellbore, with a whipstock serving as a guide. In apreferred embodiment of the invention, the power section is locatedabove the gearbox which is above the gimbal section and the flexibleshaft passes through the gimbal and drives the bit.

As the gas turbodrill is lowered downhole on the end of a pipe stringand the gas turbodrill reaches the whipstock, which has beenpre-installed, the lower turbodrill section will change direction withthe gimbal assembly providing a pivot point. As the gas turbodrill anddrill string are lowered further into the wellbore, the flex joint bendsuntil it reaches a preset bend angle limit. A highly compressed springinside of the gimbal assembly locks the bend into position.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and attendant advantages of one or more exemplaryembodiments and modifications thereto will become more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description, when taken in conjunction with theaccompanying drawings, wherein:

FIG. 1A is a is a cross-sectional view of an exemplary openhole spurlateral drilling configuration showing the steerable gas turbodrillconfigured in accordance with the invention at the start of drilling thelateral;

FIG. 1B is a cross-sectional view of an exemplary openhole spur lateraldrilling configuration showing the steerable gas turbodrill inaccordance with FIG. 1A just after the spur lateral has started;

FIG. 1C is a cross-sectional view of an exemplary openhole spur lateraldrilling configuration showing the steerable gas turbodrill inaccordance with FIGS. 1A and 1B at the completion of spur lateraldrilling;

FIG. 2A is a cross-sectional view of the exemplary steerable gasturbodrill pinch point of FIGS. 1A, 1B, and 1C.

FIG. 2B is a cross-sectional enlarged view of the pinch point of FIG.2A.

FIG. 3 is an exemplary fixed cutter bit selection chart, which ispublished by Dimatec Inc., for use in selecting a suitable cutter bitthat can be driven by the exemplary SGTD of FIG. 1;

FIG. 4 is an exemplary graph that can be used for a gas turbodrillcirculation analysis, in connection with the SGTD discussed herein;

FIG. 5A is a cross-sectional view of an exemplary gas turbodrillconfiguration;

FIG. 5B is an enlarged cross-sectional view of the exemplary gasturbodrill configuration of FIG. 5A;

FIG. 5C is an enlarged cross-sectional view of a gimbal assembly of theexemplary steerable gas turbodrill configuration of FIG. 4A;

FIG. 5D is a side perspective view of the exemplary steerable gasturbodrill configuration of FIGS. 5A, 5B, and 5C;

FIG. 6A is a side-elevational view of the exemplary steerable gasturbodrill configuration; and

FIG. 6B is a cross-sectional view of the exemplary gas turbodrillconfiguration of FIG. 6A.

DETAILED DESCRIPTION

It is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangement ofcomponents set forth in the following description or illustrated in thedrawings. The invention is capable of other embodiments and of beingpracticed or of being carried out in various ways. Also, it is to beunderstood that the phraseology and terminology used herein is for thepurpose of description and should not be regarded as limiting. The useof “including,” “comprising,” or “having” and variations thereof hereinis meant to encompass the items listed thereafter and equivalentsthereof as well as additional items. Unless limited otherwise, the terms“connected,” “coupled,” and “mounted,” and variations thereof herein areused broadly and encompass direct and indirect connections, couplings,and mountings. In addition, the terms “connected” and “coupled” andvariations thereof are not restricted to physical or mechanicalconnections or couplings.

Air drilling systems have advantages for borehole completionapplications because this technique leaves a dry, open borehole thatrequires no additional cleanout and avoids water contact with theformation.

An exemplary embodiment of an openhole spur lateral drillingconfiguration for a steerable gas turbodrill (“SGTD”) is shown in FIGS.1A, 1B, and 1C. A wellbore 10 is shown with a liner 26 supporting awhipstock 30 using liner hanger. The liner 26 and whipstock 30 may belowered into the well and oriented using a rotary drill rig or workoverrig with rotary capability which is not shown but are well known tothose skilled in the arts of drilling, well completion and wellintervention. A drillstring 24 extends from above ground into thewellbore 10 through a liner 26 that also extends from above ground andinto the wellbore. The liner 26 guides the drillstring 24 through amedial portion of the wellbore 10. The drillstring 24 and SGTD 28 arefed into the wellbore with the drill rig or workover rig using standardmethods of handling jointed tubing. Alternatively the drillstring may bea continuous length of tubing that is fed into the well with a coiledtubing well service unit also well known in the art. The SGTD 28 iscoupled to the drillstring 24 before insertion into the wellbore 10.Prior to the insertion of the SGTD 28, a whipstock 30 is run into thewellbore using liner 26. Alternatively, the whipstock 30 can be run-inseparately from liner 26 and placed with an openhole packer, not shown.The whipstock 30 serves to guide the SGTD 28 and drillstring 24 at adesired angle to thereby allow access to oil and gas bearing formationsthat are not directly downhole from the initial wellbore 10. Once it isinserted, the whipstock 30 can be directionally aligned such that thewhipstock 30 will guide the SGTD 28 in a specific radial directiondownhole. For example a wireline azimuth measurement tool can be loweredinto a wireline orientation shoe just above the whipstock 30 and theline can be rotated from surface to the desired azimuth of the whipstock30. The SGTD 28 has a gimbal joint 32 that allows the SGTD 28 to bendand change direction as the SGTD 28 is guided by the whipstock 30. Thegimbal joints 32 must allow the SGTD 28 to move through a pinch point 34when the whipstock 30 begins changing the direction of the SGTD drillbit 36.

An embodiment of the pinch point 34 in FIGS. 1A-1C is shown in greaterdetail in FIGS. 2A and 2B. The whipstock 30 incorporates a whipstockramp 38 that guides the SGTD 28 on a lateral spur into the formation.Stabilizer vanes 40 are located on the underside of the whipstock ramp38 to hold the whipstock 30 in a position in the borehole adjacent thearea intended for spur lateral drilling. The stabilizer vanes 40 aresized to slide readily into the open borehole and to allow easy rotationfor azimuthal orientation. The vanes 40 prevent lateral motion of thewhipstock 30, for example, in excess of half of an inch or a distancesignificant enough to prevent the drill bit 36 from kicking off from theborehole and into the formation. In an embodiment the whipstock 30 canfurther have a flat spring 44 connected to a ramp 46 that pushessideways against the SGTD as it passes through the whipstock 30. Theramp 46 has gradually ramped surfaces that allow the drill bit 36 totravel up or down in the borehole and slides past the ramp 46. Thespring force of the flat spring 44 is chosen to overcome the frictionalbending resistance of the gimbal joint 32. In addition, the spring 44has sufficient travel to allow the drill bit 36 to pass withoutexcessively dragging on inward facing surfaces of the ramp 46. Althougha flat spring 44 is shown, springs of other types could be substitutedto achieve a similar effect.

This system can be designed for operation off a drilling or workoverrig, which can includes the following steps:

-   -   1. Run a whipstock into the well using a liner.    -   2. Orient the whipstock using a wireline and hang in the slips,        or with a casing hanger.    -   3. Run the SGTD into the well until it reaches the whipstock.        The SGTD will bend at the pinch point on the whipstock. Pressure        applied downwardly on the SGTD by running it further into the        borehole will push the turbodrill against the whipstock and        cause the SGTD gimbal joints to bend. Additionally, in an        embodiment including the pinch point, the spring and ramp of the        pinch point will push against the gimbal joint of the of the        steerable gas turbodrill as it travels past the pitch point        thereby providing an additional force to bend the SGTD gimbal        joint. Eventually the gimbal joint will lock at a substantially        maximum angle of bend based on the configuration and internal        structure of the gimbal joint.    -   4. Drill a lateral at minimum weight on bit (WOB).    -   Cuttings are transported out of the well through the liner.

Air Compressor and Surface Equipment Pressure Capacity: For a well atwhich the SGTD will initially be employed, a current available aircompressor capacity is 1200 psig (8 MPa) @2500 scfm (70 scmm). Themaximum pressure, consistent with safe operation on air, is 2000 psig(14 MPa). These specifications are not intended to in any way belimiting on the use or functionality of the SGTD.

Exemplary Bit Design: The exemplary embodiment of the high-speed SGTDoperates with minimal torque at high speed. The SGTD may be operatedwith a variety of fixed cutter or roller come bits. In a preferredembodiment of the invention surface set diamond bits are used. Thoseskilled in the art will recognize that the maximum bit speed is limitedby thermal wear of the diamonds. The reactive torque from a surface setdiamond bit operating at maximum rotary speed is related to the WOB, W,bit diameter, D_(b), and friction, μ (about 0.4 for rock drilling)according to the following equation:

$\begin{matrix}{{M = {\frac{{WD}_{b}\mu}{3} + \frac{\delta ( {{SA} - W} )}{2\pi}}},} & (1)\end{matrix}$

where S is the drilling strength—assumed to equal the confinedcompressive strength of the rock, A is the surface area of the bit, andδ is the depth of cut per revolution. The torque will increase with rateof penetration.

An important requirement for the SGTD is to maintain well trajectorywithout any additional steering input once the drill has exited theprimary wellbore. Conventional PDM motors operating conventional fixedcutter bits generate enough torque to cause the drillstring to twist orwind up several revolutions so that it is not possible to predict theorientation of the SGTD bend while drilling. The present SGTD inventionis designed to limit the drilling torque and therefore limit the windupangle to an acceptable error level. For example if the maximum windupcan be limited to less than 45 degrees, the well azimuth can bepredicted to within this angle. If the drilling torque is known, thewindup can be predicted and accounted for when planning the well.

The windup of an example SGTD BHA and drillstring makeup is providedbelow in Table 2. The estimated torque while drilling with a 2-7/8″surface set diamond bit at about 500 lbf WOB in the Marcellus shale(15,000 psi CS) is 35 ft-lbf. The analysis is shown for 3½ or 2⅞ heavywall drill pipe. Using the larger diameter pipe cuts the windup in halfand will provide more accurate azimuthal control, however the 2⅞″drillstring may be required to accommodate return circulation. In theseexamples the total windup is 22 to 53 degrees. This amount of windup maybe acceptable or compensated for by rotating the drillstring to theright by the windup angle once the lateral is spudded, or by orientingthe whipstock to the right by the same amount.

TABLE 2 Drilling Parameters and windup angle estimates. WOB 500 LbfMotor Speed 640 Rpm ROP 50 ft/hr Shale Compressive 15000 Psi StrengthReactive Torque 35 ft/lbf Drillstring 3-1/2″ 12.95 #/ft 2-7/8″ 8.7 #/ftLength 6000 Ft OD 3.5 2.88 Inch ID 2.75 2.259 Inch windup 22 48 DegreesSGTD Whip 1-1/2″ Type CS 2.9#/ft Length 200 Ft OD 1.9 Inch ID 1.53 Inchwindup 5 Degrees Total Windup 27 53 Degrees

Exemplary Steerable Gas Turbodrill: FIGS. 4A, 4B, 4C, and 4D show anexemplary embodiment of a steerable gas turbodrill (“SGTD”) 40 for aspur lateral drilling application as shown in FIG. 1. The gas turbodrill40 includes a high-speed gas turbine power section 42 and a two stageplanetary gearbox assembly 44 in the upper section. The two-stagegearbox assembly 44 reduces the speed of turbine power section 42 outputby a factor of 12:1 and has an output shaft 46 that extends into a clampcoupling assembly 48. The gearbox assembly output shaft 46 connects to aflexible shaft 50 in the clamp coupling assembly 48. FIG. 4C shows anenlarged view of the gimbal assembly 54. When the SGTD 40 is drillingand begins to bend in the borehole, such as when coming into contactwith a whipstock, the flexible shaft 50 bends through an arc within thegimbal assembly 54. The flexible shaft 50 extends through a gimbal joint56 that enables the tool to bend through a fixed angle of up to fivedegrees. The precise angle and distance from the bend to the bitdetermines the radius of curvature of the spur lateral. The gimbal jointhas a ball and socket. The application of internal pressure plus theforce of one or more heavy springs 58, such as Belleville washer springsacting on a lock ring with a spherical seat that presses against theball. Friction between the lock ring and ball and between the ball andsocket holds the gimbal in the bent position, thereby allowing drillingof a fixed radius arc or spur lateral. The friction force is chosen toallow the gimbal joint to bend when subjected to side loads inside thewhipstock. The flexible shaft 50 couples to a bottom drill assembly 60through a flow coupling assembly 62 that extends into a bearing assemblysection 64. The bottom drill assembly 60 rotates a drill bit 64.

An alternate configuration for this tool is shown in FIGS. 5A and 5B.The gas turbodrill 150 includes a high-speed gas turbine 152 and a clampcoupling assembly 154 in the upper section, with a gearbox 156 and abearing assembly 158 located in the lower section of the gas turbodrill150. A flexible shaft 160 connects to the turbine output in the clampcoupling assembly 154 and extends through a gimbal 162 that enables thetool to bend through an angle of up to about 5 degrees. The applicationof internal pressure plus the force applied by one or more springs inthe gimbal assembly lock the gimbal in place once drilling commences.Drill bit assembly 164 couples to the gearbox output 156 and rotates adrill bit 166. Example turbine specifications are listed below, in Table4. A circulating model of the turbine in a wellbore is provided in thegraph shown in FIG. 3. Most of the pressure differential through themotor is developed through the bit nozzles. This approach reduces theturbine speed to a manageable level. The gearbox is a conventionaltwo-stage planetary design. The output torque of the motor at maximumpower is half the stall torque and this is the recommended operatingcondition. Operation at near the peak power will require WOB control towithin 100 lbf. Over weighting the bit will cause it to stall, whileunderweighting will not enable it to drill. These characteristics arecommon to turbodrills, but the relatively light weight of the JTDdiscussed herein is unique to this tool.

TABLE 4 Gas Turbodrill Turbine specifications Diameter 2-3/8″ Length 5.9ft Turbine stages 20 Gas Flow rate 2000 scfm Turbine Pressuredifferential 260 psi(1.8 MPa) Turbine Runaway speed 15000 rpm Two-stageplanetary gear reduction 12:1 Motor stall torque 70 ft-lbf Operatingspeed 625 rpm Drilling weight on bit range 300-600 lbf Operating torque35 ft-lbf Maximum Bend 5 degrees

Although the concepts disclosed herein have been described in connectionwith the preferred form of practicing them and modifications thereto,those of ordinary skill in the art will understand that many othermodifications can be made thereto. Accordingly, it is not intended thatthe scope of these concepts in any way be limited by the abovedescription.

1. A gas turbodrill system including a gas turbodrill assembly, the gasturbodrill assembly comprising: a gas turbine assembly configured tocouple to a source of pressurized fluid and comprising a drive shaft; agearbox assembly configured to couple to the drive shaft of the gasturbine and comprising an output shaft, the gearbox assembly beingconfigured to rotate the output shaft at a lower rate than the driveshaft of the gas turbine; a gimbal assembly that comprises a hollow balland socket joint that extends around a flexible shaft, the flexibleshaft configured to couple to the output shaft of the gearbox assembly,the flexible shaft extending through the hollow ball and socket joint;and a drill-bit assembly connected to the gimbal assembly, the drill-bitassembly having a shaft connected to the flexible shaft.
 2. The gimbalassembly of claim 1, further comprising at least one spring that pressesa lock ring with a spherical seat against the ball of the ball andsocket joint, the lock ring positioned to lock the position of the balland socket joint when a pre-determined force is applied and configuredto retain the locked position of the ball and socket joint when force isremoved.
 3. The gas turbodrill assembly of claim 1, further comprising apreloaded spring section in the gimbal assembly that presses against theball and socket joint and is positioned to press against the ball andsocket joint when a force is applied and assist in retaining theposition of the ball and socket joint when the force is removed
 4. Thegas turbodrill assembly of claim 1, further comprising a clamp couplingassembly enclosing the connection of the gearbox assembly output shaftand the flexible gimbal assembly shaft.
 5. The gas turbodrill assemblyof claim 1, further comprising a spring support section in the gimbalassembly positioned to tension against the flexible gimbal assemblyshaft.
 6. The gas turbodrill assembly of claim 1, further comprising adrill-bit assembly having one or more fixed cutters.
 7. The gasturbodrill system of claim 1, further comprising a drill-bit assemblyhaving a plurality of surface set diamond bits.
 8. The gearbox assemblyof claim 1, further comprising a planetary gear transmission.
 9. The gasturbodrill system of claim 1, further comprising: a whipstock positionedto guide the gas turbodrill assembly on a lateral exit path, thewhipstock having a ramp connected to a spring, the ramp and springpositioned to exert force against the gas turbodrill assembly at a pinchpoint near an upper end of the whipstock, thereby providing additionalforce to rotate the ball and socket joint of the gimbal assembly andbend the flexible shaft.
 10. A gas turbodrill system including a gasturbodrill assembly, the gas turbodrill assembly comprising: a gasturbine assembly configured to couple to a source of pressurized fluid;a gimbal assembly that comprises a hollow ball and socket joint thatextends around a flexible shaft, the flexible shaft configured to coupleto the output shaft of the gas turbine, the flexible shaft extendingthrough the hollow ball and socket joint; a gearbox assembly configuredto couple to the flexible shaft, the gearbox assembly having an outputshaft, the gearbox assembly being configured to rotate the output shaftat a lower rate than the flexible shaft; and a drill-bit assemblyconnected to the gearbox assembly, the drill-bit assembly having a shaftconnected to the gearbox assembly output shaft.
 11. The gas turbodrillassembly of claim 10, further comprising a clamp coupling assemblyenclosing the connection of the gas turbine output shaft and theflexible shaft.
 12. The gimbal assembly of claim 10, further comprisingat least one spring that presses a lock ring with a spherical seatagainst the ball of the ball and socket joint, the lock ring positionedto lock the position of the ball and socket joint when a pre-determinedforce is applied and configured to retain the locked position of theball and socket joint when force is removed
 13. The gas turbodrillassembly of claim 10, further comprising a preloaded spring section inthe gimbal assembly that presses against the ball and socket joint andis positioned to press against the ball and socket joint when a force isapplied and assist in retaining the position of the ball and socketjoint when the force is removed.
 14. The gas turbodrill assembly ofclaim 10, further comprising a drill-bit assembly having one or morefixed cutters.
 15. The gas turbodrill assembly of claim 10, furthercomprising a drill-bit assembly having a plurality of surface setdiamond cutters.
 16. The gearbox assembly of claim 10, furthercomprising a planetary gear transmission.
 17. The gas turbodrill systemof claim 10, further comprising: a whipstock positioned to guide the gasturbodrill assembly on a lateral exit path, the whipstock having a rampconnected to a spring, the ramp and spring positioned to exert forceagainst the gas turbodrill assembly at a pinch point near an upper endof the whipstock, thereby providing additional force to rotate the balland socket joint of the gimbal assembly and bend the flexible shaft. 18.A method of drilling a spur lateral well with a gas turbodrill assembly,comprising the steps of: running a whipstock into the well; positioningthe whipstock lateral exit path using a wireline; running a steerablegas turbodrill assembly into the well until it reaches the whipstock;applying force downwardly on the steerable gas turbodrill causing thegas turbodrill to press against the whipstock, thereby rotating a balland socket joint and bending a flexible shaft housed in a gimbalassembly of the steerable gas turbodrill.
 19. The method of drilling aspur lateral well as described in claim 18, further comprising the stepof: applying force downwardly on the steerable gas turbodrill withsufficient force to cause the gimbal assembly flexible shaft to bend andsubstantially lock in place.
 20. The method of drilling a spur lateralwell as described in claim 18, further comprising the steps of: applyingforce downwardly on the steerable gas turbodrill causing a ramp andspring of the whipstock to exert force against the gas turbodrillassembly near a pinch point on an upper end of the whipstock, therebyproviding additional force to rotate the ball and socket joint of thegimbal assembly and bend the flexible shaft.